The present invention relates to a device for detecting seismic motion which may be useful in preventing various disasters caused as a result of such seismic activity, and more particularly, to small-scale seismic detection devices which can be attached to various machines and devices including, for example, gas meters, oil stoves, elevators, and various components of chemical plants and railroads.
In prior art seismic detection devices of this type, a seismic detection ball is used as part of the detection mechanism as disclosed, for example, in Japanese laid-open utility model publication SN 61-48325.
In these prior art seismic motion detection devices, a seismic detection ball is placed within a casing whose bottom inner surface has a gentle spherically concave shape. In the central area of the bottom inner surface there is an additional concavity for holding the seismic detection ball in a centered position. Above the ball is a plunger having a saucer-shaped section whose bottom surface is spherically concave, having a curvature radius greater than the seismic detection ball. The concave spherical surface of the plunger contacts the top of the seismic detection ball, and is free to move up and down, following a motion of the ball. A pressure plate above the plunger is fixed at one end, and a shaft protruding from the central area of the top of the plunger is connected to the other end of the pressure plate. The tip of an activation lever for a sensing switch contacts the top surface of the pressure plate where the plunger shaft is attached.
During seismic activity, movement of the seismic detection ball within the casing pushes the plunger upward, causing the plunger shaft to upwardly displace the pressure plate, along with the activation lever, thereby closing the sensing switch. The closing and subsequent opening of the switch produces a pulse signal indicating seismic activity.
For these devices to generate the required pulse signal in response to the seismic motion, it is necessary for the seismic detection ball to be moved back and forth in a linear fashion. However, the problem with these seismic motion detection devices is that due to the spherical concavity of the inside bottom surface of the casing, the seismic detection sphere does not consistently move back and forth in a linear fashion, but may instead move in circular path, following the casing perimeter. When such circular motion occurs, the seismic detection ball maintains the plunger in an upwardly displaced position. As a consequence, the sensing switch is held in the closed position, and the expected pulse signal is not generated. This prevents the accurate detection of seismic motion.
In an attempt to solve this problem, in Japanese laid-open patent publication SN 4-52523, the present applicant proposes a seismic motion detection device having a concave bottom inner surface formed with a plurality of centrally located protrusions. These protrusions are arranged radially, sloping upward and outward. In this proposal, the rotation of the seismic detection ball is regulated in a set direction along the radial protrusions. This structure assists the ball in moving in a generally linear, back-and-forth fashion to more consistently produce the expected pulse signal in response to seismic motion, thereby allowing accurate detection of seismic motion.
These types of seismic detector devices are often installed within outdoor gas meters, or used in other similar applications where they may receive outside impacts unrelated to seismic motion. When such shocks are of great enough magnitude, the seismic detection ball within the casing may move in a circular path along the inside perimeter wall, lifted along the concave bottom wall to a position beyond the radial protrusions. Devices relying upon centrally located radial protrusions on the bottom wall are not fully effective in controlling this type of circular rotational motion.
In order to prevent circular motion of the seismic detection ball caused by an external impact, there are devices such as that disclosed in Japanese laid-open patent SN 63-263423, which have one or more protrusions formed on the inside surface of the bottom of the casing and appropriately located. Should the seismic detection ball begin to rotate along the inner perimeter casing wall, it strikes a protrusion, preventing further rotation.
Although effective in preventing circular motion of the seismic detection ball, this type of device may produce undesirable, erratic motion. By having its direction of motion abruptly altered upon striking a protrusion, the seismic detection ball may bounce back and forth, between the inner perimeter casing wall in an irregular manner before coming to rest. This erratic motion disturbs the signal waveform produced by the sensing switch and may lead to faulty detection.
Furthermore, the protrusions arranged along the inside surface of the bottom of the casing are only effective in preventing circular motion of the seismic detection ball along the casing perimeter. Rotation of the seismic detection ball along the bottom wall of the casing due to seismic motion may cause non-linear motion just as in the previously described seismic detection devices, due to the concavity of the bottom wall. Depending on the direction of rotation of the seismic detection ball, instead of moving back and forth in a linear fashion, motion may instead be non-linear or circular, preventing the accurate detection of seismic activity.
Placement of protrusions on the bottom of the casing to limit circular rotation around the casing perimeter further requires a larger diameter casing, contrary to the desired production of small-scale seismic detection devices.
In addition to the consideration of non-linear motion of the seismic ball, the prior art also addresses the need to self level such devices, in order to maintain the ball in the center of the concave bottom region in the absence of seismic activity. To accomplish this, prior art devices have been constructed to permit damped swaying of the casing, in which the seismic ball is confined within an outer housing. The casing is suspended from above by a suspension shaft fixed to the casing or to the attachment side so it can sway freely, while the other end of the suspension shaft forms a wing piece, which is disposed within in a sealed cavity filled with high viscosity fluid. In an alternative construction, the casing is arranged within the outer casing while the bottom area between the casing and the outer housing is filled with high viscosity fluid. Synthetic resin has been used as a material for constructing devices of this type because of ease of forming, low cost, etc. Because of the overall reduction in weight in these small-scale devices, there is often insufficient moment required to overcome the damper effect from the high viscosity fluid to achieve automatic centering.
In order to overcome this insufficient moment, a proposal has been made, as in Japanese laid-open utility model publication SN 3-5314, to attach a weight having a specific gravity greater than the material from which the casing is fabricated in order to lower the center of gravity. This solution, however, requires the tedious task of separately attaching a weight to the bottom of the casing. In addition, the weight must remain reliably affixed to the casing over a long period of time, despite any impact received. Because of the positioning of the weight on the casing bottom, as well as the fact that the weight and casing are made of different materials, maintaining a reliable bond is difficult.
With a current trend toward smaller and lighter-weight devices, the above described type of seismic detection device has been used widely, installed in a variety of machines. However, magnets are often present within the machines in which the devices are installed. Since steel is often used as the main component in the seismic detection balls, they may be adversely affected by the magnetic field. This results in a reduction in sensitivity or a complete failure to detect seismic activity. This undesirable effect of magnetic fields becomes more pronounced with increasing miniaturization of the seismic devices.